So now I know the dimensions I have to play with I can start to fiddle with ideas on what to make the box look like. In the following designs the black represents the carb mouth, the red the proposed box.

1 (cross section) 2 (cross section)
This would cause restriction in the intake and pose problems for the amount of air available. Rams are needed. Better in that the air is funneled in, but there is still a restriction and the “pipe” effect won’t help either.

After looking at a few more ideas it is apparent what you need is air available from all angles and that a “pipe” in itself is not much use (unless gently curved) so the box needs to be made bigger and the rams can then be put in something like design 3:


3. (cross section) 3.a (frontal)
A larger box with more air available and rams but is the space at the top large enough? A frontal view showing dimensions of the box. This is replicated on the other side to cover both carbs.

Rams are 10mm rings with 20mm in all directions before hitting the air-box wall. The top, due to restriction, only has 5mm. There is also a curvature to the top of the box to aid fitment.

In theory this would fit.